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RADIATION TECHNIQUES IN CROP AND PLANT BREEDING
MULTIPLYING THE BENEFITS
BY BEANT S. AHLOOWALIA
W
orld food
production is
based on growing
a wide variety of fruits,
vegetables, and crops
developed through advances in
science. Plant breeders have
produced multiple varieties
that grow well in various types
of soils and under diverse
climates in different regions of
the world.
Conventionally, this is done
by sexual hybridization. This
involves transferring pollen from
one parent plant to another to
obtain hybrids. The subsequent
generations of these hybrids are
grown to select plants which
combine the desired characters
of the parents.
However, another method
exists by which the genetic
make-up of a given plant variety
can be changed without crossing
with another variety. With this
method, a variety retains all its
original attributes but is
upgraded in one or two changed
characteristics. This method is
based on radiation-induced
genetic changes, and is referred
to as “induced mutations”.
During the past thirty years,
more than 1800 mutant
varieties of plants have been
released, many of which were
induced with radiation. Plant
tissue and cell culture (also
called in vitro culture) in
combination with radiation is a
powerful technique to induce
mutations, particularly for the
improvement of vegetatively
propagated crops. These crops
include cassava, garlic, potato,
sweet potato, yams, sugarcane,
ornamentals such as
chrysanthemum, carnation,
roses, tulips, daffodil, and
many fruits (e.g. apple,
banana, plantain, citrus, date
palm, grape, papaya, passion
fruit, and kiwi fruit). In some
of these plants, either there is
no seed set (e.g. banana) or the
seed progeny produces plants
which do not have the right
combination of the desired
characteristics. These
techniques are also useful in
the improvement of forest trees
having a long lifespan before
they produce fruit and seed.
This article briefly reviews
advances in plant breeding
techniques, with a view towards
improving the transfer of
technologies to more countries.
ACCELERATING
THE PROCESS
In tissue and cell culture
techniques, instead of
propagating a plant from a
seed or cutting, small pieces
or parts of the plant are
cultured under controlled
environment and aseptic
conditions on defined
nutrient media, from which
many miniature-sized plants
can be obtained. In vitro
culture of tissue has been
often called “test-tube”
culture of plants; however, the
culture containers are not
always glass test tubes. They
can be much cheaper and
versatile vessels, such as baby
food jars, jam jars, and
disposable plastic cups used
for coffee or yogurt.
Plant tissue culture —
techniques either on their own
or in combination with
induced mutations and
molecular techniques — are
used to speed-up conventional
plant breeding, and to rapidly
multiply plants in a disease-free
condition.
Plant cell and tissue culture
methods are useful for
mutation induction with
radiation as well as chemicals.
However, radiation is the
preferred method to make
genetic changes because of the
ease in treating large
populations, and the problems
of handling and disposing of
chemicals. Usually, mutations
are induced in well-known and
well-adapted plant varieties
grown in a particular region.
Selected and disease-free plants
of these varieties are cultured
in vitro, and irradiated with
gamma or X-rays. The
irradiated plants, tissues, or
cells are then multiplied many
times, also in vitro, from which
full grown plants are
produced. They are then
grown in soil for selection of
the desired types.
During the past two decades,
several in vitro culture
Mr. Ahloowalia is a staff
member of the Joint FAO/IAEA
Division of Nuclear Techniques
in Food and Agriculture.
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techniques have been
developed. Among these are:
■ Micropropagation. Small
pieces, three to five millimeters
in size, are cultured from stems
with terminal buds or from
leaves. These proliferate into
many buds and shoots which
on transfer to another nutrient
medium produce complete
plants. This procedure is the
most widely used method to
produce millions of clones
(exact copies) of plants such as
banana, strawberry, potatoes
and many ornamentals.
■ Embryo rescue. A tiny plant
embryo, which normally would
not survive during the normal
course of development in the
seed, is removed and cultured to
give a full plant. This is widely
used in the propagation of
many orchids in which the
seeds do not have sufficient
food to support the growth of
embryo to a full plant.
■ Plant regeneration. Many
plants also can be produced by
culturing cells (the building
blocks of tissues). These are
first made to grow into a lump
(callus) or are grown as cell
suspension cultures. Plants can
be produced from these cells
by changing the nutrient
medium.
In some cases, it also possible
to regenerate plants from
protoplasts (naked plant cells
from which cell walls have
been removed). Protoplasts
from different plant species
(e.g. potato and tomato) can
be fused to form somatic
hybrids which would not be
possible through sexual
hybridization. Such hybrids are
then irradiated to remove the
unwanted genetic information
of the parents.
The inactivation of cell nuclei
or protoplasts with high doses
Clockwise, from top left: Seeds of modern maize (left in above
photo, next to the undomesticated primitive type) produce higher
yielding crops. Radiation techniques can produce desirable properties
in flower colours and shapes. In-vitro, or “test tube”, techniques
greatly speed up the plant breeding process. Plant cells can be
multiplied as a callus, or lump of cells. (Credit: Ahloowalia/IAEA)
of radiation for producing
cybrids (somatic hybrids in
which cytoplasm of one cell is
fused with the nucleus of
another cell) has been also used
to produce new and novel gene
combinations; for example to
transfer cold tolerance of radish
to rape seed.
■ Anther and microspore
culture. By far, this has become
the most important in vitro
culture method used in
conjunction with radiationinduced mutations to improve
seed propagated crops. Unlike
the body (somatic) cells which
have two sets of genetic
information, the pollen grains
which are formed in anthers
have only one set of the genetic
blue-print (one set of
chromosomes), and are
haploid.
Irradiation of haploid cells
and tissues by anther or ovule
culture produces plants in
which mutated genes show up
immediately. This is because
the second set of genetic
blueprint which may hide such
mutations is absent in the
haploids. The selected haploid
plants then can be made to
produce dihaploids with the
same chromosome number as
is in the normal diploid plants.
The production of haploid
mutants and the subsequent
production of dihaploids with
mutated genes is a fast and
rapid method to produce
improved varieties in seed
propagated crops, such as
barley, rice, rape seed, maize,
and wheat. The availability of
cheap and efficient methods
for haploid production from
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the local cultivars of grain and
legume crops in developing
countries is thus essential to
utilize mutation based
breeding methods.
■ Somatic embryos. Yet
another in vitro method is
capable of producing embryos
without sexual crossing of
plants. Such embryos, called
somatic embryos, can be
produced from callus and cell
suspension cultures, and
sometimes directly on the
surface of the leaves. Somatic
embryos can be routinely
produced in many plants, e.g.
carrot, cauliflower, citrus, date
palm, potato, sweet potato. The
somatic embryos in many cases
originate from single cells or
only a few cells. Irradiation of
such cells leads to the
production of plants which
produce stable and solid
mutants. This eliminates the
need of repeated culture of
tissues to separate mutated from
non-mutated cell sectors.
MULTIPLE
APPLICATIONS
The improvement of many trees
— among which are many
important fruits such as date
palm, apple, plum, cherry,
orange, citrus — through
conventional breeding and their
propagation is a long and slow
process. Induced mutagenesis in
combination with
micropropagation can be used
for producing short height, fastgrowing types. This is even
more important for short
rotation forestry intended for
biomass production for fuel
wood and paper pulp.
Plant and cell tissue culture
techniques allow mutagenesis of
large populations of cells and
miniature plants in a small
space. Thus, it is possible to
grow millions of cells in a petridish or in a flask, and irradiate
and multiply them on defined
media to regenerate plants.
An important component of
any plant breeding programme
is the selection of desired
genotypes. The availability of
efficient and rapid methods of
selection is essential to screen
large populations of plants.
There is increasing evidence
that for some traits, in-vitro
grown cells, somatic embryos,
and miniature-sized plants can
be subjected to selection for
disease resistance, tolerance to
salinity, heat, cold and freezing.
The in vitro selected plants can
then be tested for their
performance in the field. Thus,
in vitro mutagenesis and invitro propagation can be
integrated into the
conventional selection to speed
up breeding of vegetatively
propagated plants. In vitro
culture is also important to
conserve local, well-adapted
clones which are upgraded
through mutation induction. It
also is a useful technique,
followed by irradiation and
micropropagation, for embryos
of hybrids where seed set is
extremely low. Overall, in vitro
applications widen the genetic
variability and promote
exchanges of genetic material in
the hybrids of distant parents by
recombining pieces of the
parental chromosomes.
Tissue culture often is
mutagenic by itself; plants
which originate from cell and
tissue culture occasionally do
not resemble the donor plant.
Such tissue culture induced
variation, called somaclonal
variation, can also be combined
with mutation induction as an
additional source of widening
the gene pool.
Selected mutants can be
multiplied in large numbers
and rapidly in a disease-free
condition through
micropropagation. The
technology of
micropropagation is so
advanced that now there are
over 200 commercial
companies in North America
and more than 100 in Europe
producing millions of plants
through tissue culture. In vitro
cultured plants can also be
shipped after irradiation to
countries which do not have
their own radiation facilities.
The technology of plant
tissue culture has developed at
a very fast rate. It is being used
to multiply high quality
planting material at
commercial scale in many
countries in Europe and North
America, but only a few
countries in Asia and Africa.
In many developing
countries, this technology
either does not exist or is just
in the stage of infancy. Since a
major cost component in
plant micropropagation is
labour, developing countries
can gainfully make use of
these techniques, and at the
same time create additional
employment by establishing
low-cost but high-tech based
industry for plant and crop
multiplication. In some
African countries, the only
existing laboratories for plant
tissue culture in relation to
mutation techniques have
been established with the help
of the IAEA. These
laboratories could be
expanded and used for
training and capacity building
of these countries in
micropropagation and
production of disease-free
plants.
❐
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